A single nutrient that keeps bones strong, wards off diabetes, and protects against tuberculosis, cancer, colds and the flu. Sound too good to be true? There’s more: It’s free. But you’re almost certainly not getting enough.

Research on vitamin D has flooded out the past few months, linking a growing array of health ills to low levels of the nutrient. Scientists now know that the vitamin, which is naturally produced in skin exposed to the sun’s ultraviolet rays, binds to cell receptors throughout the body and that a lack can cause various systems to malfunction.

In December 2006, for example, University of Pittsburgh researchers reported that a D deficiency doubles the risk of dangerous hypertension during pregnancy because the nutrient helps control a hormone affecting blood pressure. In March 2006, a study examining how the vitamin affects the pancreas’ release of insulin found the risk of diabetes to be one-third lower in people with the highest levels than in those getting the least.

“The vitamin D story is becoming clear. I think it’s very exciting,” says Robert Heaney, a professor of medicine at Creighton University in Nebraska who’s researched the nutrient’s effects on the bones and who, like many researchers, now thinks supplements are a good idea.

Prior to the industrial revolution, humans had no trouble getting an abundance of the sunshine vitamin; a mere 10 to 15 minutes outdoors at midday gives the average fair-skinned person 10,000 international units.

That’s far above the government’s dietary recommendations of 200 IUs a day up to age 50, 400 IUs to age 70, and 600 IUs over 70. But most people nowadays spend little time outdoors, and food sources such as milk and salmon contain relatively modest amounts. What’s more, the rash of new findings suggests to the experts that the guidelines are way too low.

“There’s no one working in the field who thinks these levels still make sense,” says Walter Willett, a professor of epidemiology and nutrition at Harvard University whose recent studies have focused on the connection between vitamin D and cancer.

Many people run particularly short during the winter, says vitamin D researcher Michael Holick, a professor of medicine at Boston University School of Medicine. That’s because anyone living north of Atlanta makes little, if any, from the sun when the UV rays fall at too low an angle to penetrate the atmosphere.

Vitamin D is best-known for promoting bone health. It was first added to the milk supply in the 1930s to prevent the bone-deforming disease rickets, and it defends against osteoporosis by triggering the absorption of calcium into bone cells.

New evidence indicates that many people suffering symptoms of chronic fatigue syndrome and fibromyalgia actually have a painful softening of the bones caused by a D deficiency.

But having too little appears to cause the immune system to weaken. A landmark study published in the March 2006 issue of Science found that cells from African-Americans (whose dark skin doesn’t efficiently absorb UV rays) churned out 63 percent less of a protein needed to fight off tuberculosis than expected. When added to the cells, vitamin D appeared to signal the cells to produce normal levels of the protein.

An immune-system link might explain why the flu seems to strike only during the winter. A review of more than 100 studies on vitamin D and respiratory diseases, published in a recent issue of Epidemiology and Infection, found that low levels probably allow the viruses to penetrate the immune system.

“It’s the first comprehensive theory set forth to explain the seasonality of influenza,” says vitamin D expert and lead author John Cannell, president of the Vitamin D Council and staff psychiatrist at Atascadero State Hospital in California. What’s now needed, he says, is a trial to see if those exposed to flu viruses are less likely to come down with an infection if they take supplements.

The possibility intrigues researchers bracing for an outbreak of avian flu, which quickly kills by triggering an excessive immune response. Victims often suffocate when an onslaught of disease-fighting cells, known as a cytokine storm, results in a rapid buildup of fluid in the lungs.

Experts think vitamin D might rev up the part of the immune system that prevents the germs from gaining entry to cells in the first place.

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